new, they're in the $500-600 range. i was lucky to find a VERY used one for $350 that i talked down to $275. it had some rust on it, and the blueing was about 40% on the receiver. i plan on refurbishing it and turning it into a tactical shotgun.
Prices on new guns are pretty static, used guns depend a bunch on location..
Down here, the place to buy new is Wal-Mart - ours has 11-87s for about 500.. Decent used 1100s go for $275 and up. No great trick to find one at $300 - 350 ANIB.